The invention refers to a serial printing device able to allow a printing head to automatically position itself at a set distance from a contrasting element, according to the thickness of the document introduced into the printer. The document can consist of a sheet of paper, thick or thin, or a booklet composed of several pages, such as for example a passport, a post office savings book, a bank book, a data processing table or suchlike. The printing head can be of the impact type, for example with needles, or without impact, for example of the ink-jet type.
In serial printers, especially but not only impact-type printers, one of the most serious technical problems is to take the writing element, whether it be a printing head with needles, ink-jet or otherwise, to a set distance from the printing support; this distance is usually in the range of several tenths of a millimeter.
The state of the art includes printing devices wherein the contrasting element, or platen, is assembled in a fixed position on a supporting structure, while the writing assembly, or writing element, is assembled on a carriage which can move with respect to the platen, drawing nearer to it or moving farther from it, according to the thickness of the printing support.
The state of the art also includes devices wherein, vice versa, the writing element and the carriage have a fixed position with respect to the writing plane, and it is the platen which can move, backwards and forwards, with respect to said writing plane.
Apart from printers with a low quality level, where the operator can vary the distance manually, in high-quality professional printers this operation is carried out automatically by means of sensors, sometimes even sophisticated and expensive ones, which detect the thickness of the document introduced between the contrasting element and the writing element. These sensors are assembled either in a fixed position or on board the printing carriage.
Moreover, since the document, as in the case of booklets, could have a variable thickness within the same printing line, before printing a line, the printing carriage on which the sensor is assembled is sometimes made to translate to make it perform a service travel along the entire printing line, in order to detect and memorize the possible variations in thickness. In a subsequent travel, possibly made backwards, the carriage or platen, by means of complex and expensive servo mechanisms, are displaced with respect to each other, according to the thickness detected.
Until today, at least in the field of office printers, no-one has ever invented a serial printing device which will allow the writing element and the platen to vary their reciprocal position, simply and automatically, according to the thickness of the printing support, also along the same printing line.
Document JP-A-63-188074 discloses a printing device comprising a printing head mounted on a carriage and pressed by a first spring towards a stationary reference rail disposed parallel to a platen. When the carriage moves parallel to the platen, the printing head also slides on the reference rail parallelly to the platen. The latter has a circular surface which is pressed towards the stationary reference rail by a second spring. The platen, against the action of such second spring, displaces with respect to the reference rail corresponding to the thickness of the printing paper interposed between the platen and the same reference rail.
Document JP-A-63-188075 discloses a printing device comprising a printing head mounted on a carriage and pressed by a first spring towards a stationary reference rail. A pair of rollers are connected therebetween by means of a lever and are disposed parallel to the reference rail. When the carriage moves parallel to the rollers, the printing head also slides on the reference rail parallelly to the rollers. A first of such two rollers is pressed towards the reference rail by a second spring. The two rollers, against the action of such second spring, displace with respect to the reference rail corresponding to the thickness of the printing paper interposed between the first roller and the same reference rail.
Both these two above-mentioned printing devices have the disadvantage not to permit the printing head to displace with respect to the platen/rollers to adapt perfectly to the variations in thickness, even within the same line, of the printing document, particularly when the latter consists of a booklet composed of several pages, such as for example a passport, a post office savings book, a bank book or suchlike.
The present Applicant has devised and embodied the serial printing device according to the invention to overcome the shortcomings of conventional devices and to obtain further advantages, which are set out hereafter.
The invention is set forth and characterized in the main claim, while the dependent claims describe other innovative characteristics of the invention.
The main purpose of the invention is to achieve a device which will enable a serial printing element to adapt perfectly to the variations in thickness, even within the same line, of the document it has to print on, especially in the case that the latter consists of a booklet composed of several pages, such as for example a passport, a post office savings book, a bank book or suchlike, without using sophisticated sensors or servo mechanisms.
In accordance with this purpose, the printing device according to the invention comprises a fixed structure on one side of which a printing carriage, able to slide along the printing line, and on the other side a contrasting element are assembled. A printing element is in turn assembled on the carriage. According to a characteristic feature of the invention, a reference element is associated with the printing element, and is able to contact the surface of the paper support to be printed on, first elastic means are disposed between the printing element and the carriage and second elastic means are disposed between the contrasting element and the fixed structure.
The reference element is disposed very near the writing end of the printing element and substantially coplanar thereto. Consequently, the reference element, coming into contact with the surface to be printed on, in fact also determines the distance between the latter and the printing element.
The reference element advantageously comprises a wheel solid with the printing element, so that its cylindrical surface is substantially tangent to a writing plane which also passes through the writing ends of the printing element.
The parameter of rigidity of the above-mentioned first elastic means is less than that of the second elastic means, so that when the thickness of the documents which have to be printed on is contained within a set value, only the first elastic means are suitable to yield. In this case it is only the printing element which moves with respect to the carriage, while the contrasting element remains still. On the contrary, when the thickness of the document exceeds this set value, the second elastic means also yield and consequently the contrasting element too is distanced from the printing element with respect to its initial inactive position.
The device is therefore self-adapting and requires neither sensors nor servo mechanisms to regulate the distance of the printing element from the contrasting element, that is, the head from the platen.
Moreover, the different thicknesses of the document to be printed on are immediately recognized, even within the same printing line, allowing the device according to the invention to treat any type of document.
There is therefore no constraint either on the positioning of the document introduced nor on the size of the document.
The fact that the contrasting element too can move elastically after the printing element has already made a certain travel means that the possible introduction of a particularly thick document, such as for example a passport, does not damage the printing element in the least, because the difference between the thickness of the document and the above-mentioned travel is absorbed by the yielding of the contrasting element.
The device according to the invention also obtains a breaking of the critical printing frequencies, with a consequent reduction in noise, and also rapidly damps the oscillations, due to the displacement of the two mechanical elements (printing element and contrasting element).